Hi Olivier,
if you are using the official FlightAware installation you could easily add the Opensky feeder by following this guide
OpenSky feeder installation guide
. FlightAware performs some calculations for multilateration (MLAT) on your Raspi. If you are adding several other feeders doing the same, you may mix MLAT data causing strange results on your web interface. The OpenSky feeder doesn't perform MLAT calculations and does not disturb your system in that way.